Triterpenoid saponins are widely distributed in legume crops (9, 52). For example, soybean seeds contain on a dry weight basis about 0.5% of types A and B soyasaponins, depending on variety, cultivation year, location of growth and degree of maturity (55). The main saponins of soybeans (more than 20 have been identified) are glycosides of soyasapogenol B, the aglycone of soyasaponin I (S-I). Although S-I was initially thought to be the principal ingredient, it is currently believed that S-I does not exist as a free alcohol (C-22 OH) in soybeans but is formed during extraction from the naturally-occurring soyasaponin VI (S-VI), also known as soyasaponin βg, soyasaponin BeA or chromosaponin I (35, 40). The hydrolytic reaction occurring at C-22, generating 3-hydroxy-2-methyl-4-pyrone (maltol) and S-I, is reputed to occur gradually in solution, is promoted by heat and is catalyzed by alkaline conditions. This type of conjugation has been demonstrated to occur in other legumes, including pea seedlings (Pisum sativum L.) and mature chickpea (Cicer arietinum L.) seeds (29, 65). In 1996, levels of S-VI in two cultivars of chickpea were estimated at 0.075% (desi type) and 0.071% (kabuli type) on a dry weight basis (53). Earlier, a sample of chickpea was reported to possess levels of soyasapogenol B of 0.075% (24). Varietal differences and effects of processing on saponin content of chickpea have also been studied (27). The biological and pharmacological activities of saponins have been reviewed (36).
Although field peas were initially thought to contain S-I (and then S-VI) as the only soyasaponin, we recently showed that field pea extracts contained dehydrosoyasaponin I (D-I) as a minor component. D-I isolated in small quantities from pea had insecticidal and antifeedant properties against stored product insect pests (59, 60, 61). This C-22 keto saponin, with soyasapogenol E as the aglycone, had also been isolated as a minor component from immature green pea seeds (45). D-I was also known to occur as a minor component in soybeans (35) and other legume species (8, 30, 32-34, 41, 43, 44, 70) but not in chickpea. D-I isolated from leaves of Desmodium adscendens was shown to be a high-affinity activator of calcium-dependent potassium channels and was 60 fold more potent than S-I as a potassium channel opener (41, 42, 46). The occurrence of D-I in leaves of D. adscendens, a popular herbal medicine used in some countries such as Ghana, might explain the medicinal value of this herb as a treatment for asthma and other conditions associated with smooth muscle contraction (1). Since the concentration of D-I in all known sources was very low, better sources of D-I are needed to explore its medicinal value.
We have shown here that aqueous alcohol extracts from the chickpea (Cicer arietinum L.) were capable of yielding a rare but highly valuable triterpenoid saponin dehydrosoyasaponin I (D-I) in relatively high yields. D-I is a valuable natural product because it is known to be a potent calcium-activated potassium channel opener (41). Agents that modulate potassium channels are emerging therapeutic drug targets for treating cardiovascular, urological, respiratory, neurological and other disorders (38, 46, 47). The development of D-I for these and other medicinal applications has been severely hampered because an adequate supply of D-I has not been available. Thus, there is a need in the art for plant compositions enriched in dehydrosoyasaponin I (D-I) and also novel methods of producing compositions enriched in dehydrosoyasaponin I (D-I). Further, there is also a need in the art for novel methods of producing other soyasaponins, for example S-I as described herein.